Differential regulation of UDP-GlcUA transport in endoplasmic reticulum and in Golgi membranes

Background: In the endoplasmic reticulum (ER), the stimulation of UDP-glucu ronosyltransferase (UGT) by UDP-GlcNAc is based on the interaction of trans port across the ER membrane of UDP-GlcUA with UDP-GlcNAc. Intramicrosomal U DP-GlcNAc stimulates influx of UDP-GlcUA and thereby enhances delivery of U DP-GlcUA to the catalytic center of UGT in the ER lumen. Aim: The aim of this study is to investigate whether the interactions betwe en nucleotide sugars for transport across the ER membrane also occur in the Golgi apparatus, and thereby affect UGT activity in Golgi membranes. Results: We found that Golgi membrane preparations display UGT activity whi ch, unlike in ER membranes, is not stimulated by UDP-GlcNAc, Efflux of intr avesicular UDP-GlcNAc and UDP-Xyl marginally enhanced uptake of UDP-GlcUA i n Golgi vesicles; such trans-stimulation was much more pronounced in the ER , Efflux of intravesicular UDP-GlcNAc was strongly trans-stimulated by cyto solic UDP-GlcUA in ER-derived vesicles but less so in Golgi-derived vesicle s. Conclusion: The interaction between transport of UDP-GlcUA and transport of UDP-GlcNAc or UDP-Xyl is different in Golgi vesicles compared with ER vesi cles. This finding is consistent with the different effects of UDP-GlcNAc o n glucuronidation in Golgi and ER. (C) 2001 European Association for the St udy of the Liver. Published by Elsevier Science B.V. All rights reserved.